Full-Chain Membership Proofs (FCMP) – The Next Generation of Monero's Privacy

This is going to have very interesting implications.

The article here: https://www.getmonero.org/2024/04/27/fcmps.html compares it from having an anonymity set of 12 to 1,000,000.

What does "chain" mean in this context? I would think chain meant the entire blockchain however 1,000,000 seems like a small number to use then (unless there have only been that many monero transactions total?).